Exemplary embodiments relate to a method of programming a nonvolatile memory device.
There has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and can retain its data even without the supply of power. Further, high-capacity memory devices that are highly integrated and capable of storing a large amount of data are desired. Such a nonvolatile memory device may, for example, be a plurality of memory cells coupled in series together to form one cell string, where each memory cell array includes a plurality of the cell strings.
Each cell string of the nonvolatile memory device has a structure in which a plurality of memory cells are coupled between a bit line and a source line. Through such a string structure, the number of contacts between the bit line and the source line can be reduced and the size of the memory cell can be made small, thereby enabling high-capacity memory. Such a string structure, however, raises a concern in that the access speed is low because the cell current of a memory cell becomes smaller with a reduction in the size of the memory cell.
In a multi-level cell, unlike a typical memory device, one memory cell of the nonvolatile memory device can store 2 or more bits of data (i.e., a multi-level form). That is, the memory cell can store data of various states by having multiple distributions of threshold voltages by controlling the amount of charges trapped at the floating gate of the memory cell. Such a multi-level cell (MLC) program method is being widely used because it can significantly increase the memory capacity of the memory cell. In the multi-level cell (MLC) program method, data of two or more logical pages are programmed into one physical page. However, the time that it takes to perform a program operation is relatively long and a process of performing the program operation is relatively complicated, because data of several logical pages are programmed into one physical page. Furthermore, with an increase in the number of threshold voltage distributions, the width between the distributions of the threshold voltage is narrowed. Accordingly, the read margin is reduced, which raises a concern in reading data accurately.